Filtering device for filtering out supply voltage interferences

ABSTRACT

A filtering device for filtering out a supply voltage interference of a voltage source that delivers a supply voltage for an electrical load, in which the filtering device includes a capacitor that establishes a connection between the voltage source and the load to a reference potential, and in which a diode is connected between the voltage source and the capacitor, to which a variable resistor is connected in parallel.

FIELD OF THE INVENTION

The present invention relates to a filtering device and method forfiltering out supply voltage interference of a voltage source whichdelivers a supply voltage for an electrical load, having a capacitorwhich establishes a connection between the voltage source and the loadto a reference potential.

BACKGROUND INFORMATION

A filtering element having a coil and a capacitor and referred to as anLC filter may be used for suppressing line-conducted systeminterference. It should ensure that no interference, for example, of amotor vehicle generator or another vehicle component can be heard in anaudio signal of a car radio, for example. In order to protect a load tobe connected from damage due to accidental polarity reversal, a powerdiode may also be provided for tripping a fuse in the event of polarityreversal.

The aforementioned LC filter is, however, an oscillating system whoseresonance frequency is in the range of a bass frequency of audioreproduction. In the event of resonance, the energy stored in thecapacitor's coil is moved back and forth between these two componentsand thus may not be transmitted to the load, i.e., an LF power outputstage of a car radio or a stereo system, for example. As a result, theoutput stages of car radios, for example, may not be supplied withsufficient bass frequency current, resulting in an increased distortionfactor in the bass range, which is referred to as “bass bulge.”

It is believed that another disadvantage of the use of such an LC filteris that the effect of the coil in the case of high load currents, i.e.,in the case of a high audio output power of a car radio as a load, forexample, may diminish considerably, so that in this case only an ohmicresistance and a highly reduced reactance of the coil remain effective.In this case, an AC current load on the capacitor and thus its powerloss may become excessive, resulting in accelerated aging of thecapacitor. The magnitude of the load depends on the distribution of thecurrent flowing through the coil resistance and the internal resistanceof the capacitor. Accordingly, if the internal resistance of the coil islow, the load on the capacitor is similarly low.

SUMMARY OF THE INVENTION

An object of an exemplary embodiment and/or exemplary method of thepresent invention is to provide an improved filtering device of theabove-named type and a corresponding method for operating it, whichwould eliminate the above-named disadvantages and achieve improvedfiltering of interference while affecting the load as little aspossible.

It is believed that this objective may be achieved with a filteringdevice of the above-named type, in which a diode is connected betweenthe voltage source and the capacitor, and to which a variable resistoris connected in parallel.

It is believed that this has the advantage that a coil-less filter ismade available, which eliminates the disadvantages introduced by thecoil in the systems referred to above, while its operation is notaffected and is therefore improved, since there is no oscillatingsystem.

An aspect of an exemplary embodiment of the present invention is thatthe coil is replaced in the filtering element by a diode, which isbridged by a controlled resistor in the event of high load currents. Atthe same time, the polarity reversal protection diode may be omitted, sothat, in the event of accidental polarity reversal when the load isconnected, the fuse is not destroyed. Since the circuit according to theexemplary embodiment of the present invention is not an oscillatingsystem, it is believed that all disadvantages associated with suchsystems may be eliminated, since there is no more resonance or bassbulge.

Since the capacitor cannot be discharged via the power supply or thevehicle electrical system, which may occur in the case of an LC filter,it is believed that the undesirable phenomena associated therewith suchas clicking noise due to an audio output stage may be avoided.

In an exemplary embodiment, the variable resistor is an electricallycontrollable resistor, the diode and the resistor being arranged as aMOSFET transistor connected between the voltage source and thecapacitor, which is provided with a control circuit for controlling thevariable resistor.

In order to ensure sufficient power supply to the load in the event ofhigh power demand, the control circuit for deriving a control signal forthe variable resistor is connected to a power output of the load, forexample.

The voltage source may be a motor vehicle electrical system and the loadmay be an automotive audio component, a car radio in particular. Thereference potential may be a ground potential, for example.

In operating the exemplary filtering device according to the exemplaryembodiment of the present invention, the resistance of the variableresistor is reduced as the power consumption of the load increases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary embodiment of a filtering device according tothe present invention.

FIG. 2 shows an exemplary circuit of an exemplary embodiment of afiltering device according to the present invention.

DETAILED DESCRIPTION

According to the schematic block diagram of FIG. 1, a filtering device100 according to an exemplary embodiment of the present inventionincludes a diode 10, which is connected between a terminal 12 of avoltage supply (not shown) and a terminal 14 of a load (not shown), avariable resistor 16, which is electronically controllable inparticular, connected in parallel to diode 10, and a capacitor 18, whichconnects diode 10 and terminal 14 on the load to a reference potential20, such as ground potential.

A control circuit 22 is provided to set a resistance value of variableresistor 16. Control circuit 22 derives an appropriate control signalfor setting the resistance value of variable resistor 16 from a signaloutput of the load. If the load is a car radio, for example, the controlcircuit may be connected to an audio output of an LF output stage of thecar radio. In the event of high load currents, which corresponds to ahigh LF level, for example, diode 10 is bridged by variable resistor 16due to a corresponding reduction in the resistance of variable resistor16. The resistance of variable resistor 16 is reduced so that it islower than the resistance of a coil that would otherwise be used. Thisreduces the AC current load and thus the power loss of capacitor 18. Dueto the low voltage drop across the bridge resistor, a higher voltage isavailable to the load, for example, to the audio output stages of a carradio, so that an output power of the load is higher compared with anarrangement having LC filters under conditions that are otherwisecomparable, such as system voltage, LF total harmonic distortion, etc.In addition, by replacing the large coil with a relatively small diodethe dimensions of the filtering device are reduced, which is anincreasingly important factor for the compactness of today's car radiosand other automotive components, for example.

As long as only diode 10 is active in operation, load capacitor 18 ischarged to the maximum value of the supply voltage affected by theinterference in the case of supply voltage interference. The ripplecontent of the capacitor voltage is determined only by the currentconsumption of the load. As soon as an output power of the load, e.g.,the LF output power of a radio, becomes maximum, diode 10 is bridged byvariable resistor 16, so that no voltage drop takes place across thediode, which would limit the control range.

Since in the case of automotive audio components such as radios orboosters, a relatively low quiescent current flows, which increases withincreasing LF power, in an exemplary embodiment of a filter deviceaccording to an exemplary embodiment of the present invention, theresistance of variable resistor 16 is directly gated with an LF outputsignal, as illustrated in FIG. 2. Here the combination composed of diode10 and variable resistor 16 is implemented by a power MOSFET transistor26. Such a MOSFET transistor 26 already contains a power diode 10, sothat both elements, diode 10 and variable resistor 16, are combined in asingle housing. The device is controlled via a charge pump, whichgenerates a required control voltage as a function of an output powerdirectly from the audio signal at audio outputs 24.

As an alternative, variable resistor 16 is controlled as a function ofother control quantities derived from the audio signal.

What is claimed is:
 1. A filtering device comprising: a capacitor forestablishing a connection between a voltage source and a load to areference potential, the voltage source delivering a supply voltage foran electrical load; a diode connected between the voltage source and thecapacitor; and a variable resistor connected in parallel to the diode,the variable resistor being controllable as a function of a powerconsumption of the load.
 2. The filtering device of claim 1, wherein thevariable resistor is controllable so that a resistance of the variableresistor decreases as the power consumption of the load increases. 3.The filtering device of claim 1, wherein the variable resistor iscontrollable by a control circuit.
 4. The filtering device of claim 3,wherein the control circuit is operable for deriving a control signalfor the variable resistor, and the control circuit is coupled to a poweroutput of the load.
 5. The filtering device of claim 1, wherein thevoltage source is a motor vehicle electrical system and the load is atleast one of an automotive audio component and a car radio.
 6. Thefiltering device of claim 5, wherein the control circuit is operable forderiving a control signal for the variable resistor, and the controlcircuit is connected to an audio output of an LF output stage of theload.
 7. The filtering device of claim 1, wherein the referencepotential is a ground.
 8. The filtering device of claim 1, wherein thediode and the variable resistor are provided by a MOSFET transistorconnected between the voltage source and the capacitor.
 9. The filteringdevice of claim 1, wherein the filtering device is for filtering out asupply voltage interference in the supply voltage deliverable to theload, the voltage source generating the supply voltage interference. 10.A method of operating a filtering device, the filtering device includinga capacitor for establishing a connection between a voltage source and aload to a reference potential, the voltage source delivering a supplyvoltage for an electrical load, a diode connected between the voltagesource and the capacitor, and a variable resistor connected in parallelto the diode, the method comprising the step of: controlling thevariable resistor as a function of a power consumption of the load, aresistance of the variable resistor decreasing as a power consumption ofthe load increases.